The search for the Kim family is the latest example of how important cell phone technology has become as a public safety tool.

While other technologies such as global positioning system, or GPS, navigation may help people find their way out of trouble, it does little to help when people are stranded on the side of the road like the Kims were. Tracking devices that send beacons to rescuers could be helpful, but they are used mostly by wilderness backpackers and backcountry skiers. Few people carry them on road trips.

At the end of the day, the technology that has proved the most valuable for locating lost or missing people has been cellular phones. Even General Motors’ OnStar service, which provides GPS navigation and tracks cars when they are stolen, relies on a cellular network to communicate with the GPS receiver in the car.

So how does it all work? Mobile devices, when they are within range, constantly let cell towers and the mobile switching center, which is connected to multiple towers, know of their location. The mobile switching center uses the location information to ensure that incoming calls and messages are routed to the tower nearest to the user.

If a subscriber is unable to get service, this location information is usually purged from the mobile switching center. But some location information may remain in call detail records. Some mobile operators may store the most recent communication between a device and a mobile switching center for a certain period of time, usually 24 hours.

When someone is missing, even this small bit of information can prove useful in determining the approximate location of a device using the updates from the mobile switching center. If the mobile subscriber is still within cell phone range, authorities can track his or her general movement by following the sequence of towers the phone has contacted or pinged. And if the cell phone goes out of range or runs out of battery power, the mobile operator may be able to use the last recorded location before the cell phone either lost its signal or lost power.

But the most useful information for locating people when they are lost comes when someone has initiated or received a call or text message on their phone. Mobile operators keep records of these events for billing purposes in what is known as a call data record, or CDR. And they can go back to these records to get a historical account of the cell phone’s location.

This is actually what authorities used to locate the Kims’ phone, according to Eric Anderson, director of engineering for Edge Wireless, a regional mobile operator that provides cellular phone service in the area where the Kims were stranded.

One of Edge Wireless’ cell phone towers briefly connected with one of the family’s phones at about 1:30 a.m. November 26 near Glendale, Ore. The phone was connected long enough to the network to send a notice that there was a voice mail or text message waiting. But the connection didn’t last long enough for the Kims to retrieve the message or initiate a call for help.

Still, the connection was long enough that two Edge Wireless engineers, Eric Fuqua and Noah Pugsley, were able to find this information in the CDR to determine that the family was in sector “Z” in the southwestern portion of the cell site’s 26-mile radius. Wolf Peak’s “Z” sector provides coverage to remote areas with little population and very little cell phone traffic. Using this information, authorities sent out rescue teams, which eventually located Kati Kim and her children.

“I spotted her waving the umbrella and running around the road” next to their silver Saab station wagon, Rachor said. Someone had stamped out “SOS” and “Out of gas” in the snow. Rachor radioed in her position. Within minutes, Rice said, the three Carson helicopters were there. One crew dropped food, and a smaller one was able to land and pick them up.

According to Unstrung, each of the methods used to obtain location information has its own pros and cons. Operators usually choose a variation of one or more of the systems, depending upon which application best suits the legacy network already in place.

The UK’s Celldar project allows surveillance of anyone, at any time and anywhere there is a phone signal. The technology ’sees’ the shapes made when radio waves emitted by mobile phone masts meet an obstruction. Signals bounced back by immobile objects, such as walls or trees are filtered out by Celldar gear. This allows anything moving, such as cars or people, to be tracked. Passively. It can’t identify anyone — it simply displays moving blobs. Like radar.